Effect of Rare-earth Oxides on the Formation, Densification and Property Development of Magnesium Aluminate Spinel Prepared from Different Oxide Reactants in a Single Stage Firing Process

Abstract

Magnesium aluminate (MgAl2O4) spinel has a high melting point (2135 oC), high strength at room and elevated temperatures, low thermal expansion coefficient, high thermal shock resistance, and high chemical resistance; these properties are very useful during service conditions. MgAl2O4 is an important refractory material for cement rotary kilns, steel teeming ladle linings, regenerator checker-work bricks for glass tank furnaces, etc. Despite these advantages, spinel is difficult to sinter since its formation is accompanied with a volume expansion of ~5-8% due to the density differences between the reactant oxides, MgO and Al2O3. Single-stage firing may densify the material poorly, limiting its applicability in refractory applications. Two-stage sintering improves spinel product densification and characteristics, but it is too expensive and time-consuming for large-scale production. Among the various methods for spinel synthesis, solid-state reaction (SSR) route is regarded as the most cost-effective and easy method for bulk manufacturing from an industrial standpoint. Additives are foreign substances that are deliberately added to a system to aid in processing and improve its resultant properties. The effect of different additives, predominantly compounds of alkali/alkaline earth or transition metals, on the formation and densification of spinel has already been explored extensively. Rare-earth oxides (REOs) are characterised by useful combination of properties such as high melting point, high thermal stability, chemical inertness, etc. and therefore have a great potential to serve as key additives in the development of spinel refractories. The number of studies on the synthesis of magnesium aluminate spinel via a single-stage firing method and use of REOs as additives on reaction sintering of spinel, and their effects on mechanical and thermal shock behaviour are limited, leaving room for further research. Also, only a handful of studies have adhered to the continuous incremental addition of REOs. Intermediate milling prior to sintering stage improves spinel properties, however it may add to synthesis costs and hinder commercialization. Use of commercial grade raw materials for producing in-situ spinel (preformed spinel are otherwise expensive) and use of a single-stage solid state reaction sintering process can offset the cost of REO-doped spinel. Additionally, if the amount of REO addition can be maintained low relative to bulk spinel, the overall cost of such materials can be reduced. Therefore, the current work was performed to discern the role of incremental addition of each of the rare-earth oxides (Y2O3, Sm2O3 and Dy2O3), separately, between 1 to 4 wt. %, on the reaction sintering of various magnesium aluminate spinel batches using different commercial-grade oxides and employing a single-stage solid-oxide reaction sintering route without an intermediate milling step. Initially, a stoichiometric batch of spinel was prepared, and then the effect of each of the REOs on the densification, phase formation, microstructure, and mechanical properties such as flexural strength and strength retained after thermal shock of the spinel product were also investigated. The study showed that 2 wt. % Y2O3 spinel batches showed the highest densification. Y2O3-containing batches showed garnet (YAG, Y3Al5O12) phase at all sintering temperatures. Y2O3 doping densified spinel due to YAG's same cubic crystal structure isotropy with that of formed spinel. Microstructural study showed that 2 wt. % Y2O3 containing batches sintered at 1650 oC have a controlled grain structure. They also revealed better strength and thermal shock behavior than undoped spinel batches. In a similar fashion, for the samarium oxide and dysprosium oxide-based spinel batches, 1 wt. % was obtained as the optimized concentration level. Phases samarium aluminate (SmAlO3) and dysprosium aluminum garnet (DAG, Dy3Al5O12) were found in the Sm2O3 and Dy2O3 containing spinel batches, respectively, at all sintering temperatures which helped in the process of densification, and led to improved mechanical properties. Overall, among all the rare-earth oxides used in the present study, 1 wt. % Dy2O3 showed maximum improvement in property development.